Model declarations, ToolDefinition export, and minor tool fixes

[Kamilbenkirane]

Priority: Medium
Related: #199

Rollup of medium-priority items:

• ToolDefinition not exported: Type defined in tools.py:54 but not in __all__ or __init__.py — users can't type-hint their tools lists
• Inconsistent error messages: Some say "not supported by Anthropic", others "not supported by this provider" — standardize
• ToolCall.arguments typing: Uses dict[str, Any] instead of dict[str, JsonValue]
• ToolSupport empty list UX: Error says "Supported: []" — should say "Model does not support any tools"

Files:

• src/celeste/tools.py:54
• src/celeste/__init__.py
• src/celeste/protocols/chatcompletions/parameters.py:101
• src/celeste/providers/anthropic/messages/parameters.py:94
• src/celeste/types.py:41
• src/celeste/constraints.py:386